1. Field of the Invention
The present invention relates to a laser beam printer for forming a copy image on a photosensitive member with a laser beam and transferring the image onto record paper.
2. Description of the Prior Art
Conventionally, such laser beam printers primarily comprise an optical system for emitting an image forming beam, a photosensitive member for forming an electrostatic latent image thereon with the beam and a developing system for developing the latent image. A partition wall is interposed between the optical system and the developing system for preventing the optical system from contamination with dust or the developer spilling from the developing system. The laser beam from the optical system impinges on the photosensitive member in the developing system through a dust filter provided on a window of the partition wall to form images on the member (see, for example, U.S. Pat. No. 4,469,430).
An optical filter, e.g. a transparent glass plate, is used as the dust filter since such a filter is simple in construction, easy to install and also easy to clean or replace when soiled.
However, the use of the optical filter involves the problem that the laser beam passing through the filter is partly reflected from the surface of the filter and then reflected from a polygon mirror again to form a ghost image on the photosensitive member.
U.S. Pat. No. 4,171,902, for example, discloses a technique for removing ghost images, but this prior-art technique is intended to remove a ghost image attributable to the light reflected from the photosensitive member and is not adapted to remove the ghost image to be formed by the light reflected from the surface of the above-mentioned optical filter.